The present invention relates to a signal converter circuit for a color TV. More particularly, the present invention relates to a signal converter circuit for converting a color TV signal of the NTSC system which has a carrier chrominance signal obtained from quadrature modulation of the subcarrier by two chroma signal components into a color TV signal of the
system which is obtained from quadrature modulation by phase alternation by lines, or vice versa. In a video disc reproduction system or the like for recording NTSC-system carrier chrominance signals by low-frequency conversion and for reproducing them, the signal converter circuit of the present invention may be suitably adapted for reproducing the signals with a PAL-system TV receiver.
In general, an NTSC color TV signal includes a carrier chrominance signal obtained by quadrature modulation of the subcarrier by two chroma signal components, in addition to the luminance signal. When such a color TV signal is to be recorded on a recording medium such as a video disc, the carrier chrominance signal is low-frequency converted, and the obtained low-frequency signal is recorded. In order to reproduce the signal from the video disc, frequency conversion of the signal to the original frequency carrier chrominance signal is performed.
The reproduction system of a video disc on which signals are recorded by the NTSC system will briefly be described with reference to FIG. 1.
A color TV signal to be reproduced and detected from a video disc is separated by a filter into a carrier chrominance signal Ch and a luminance signal (Y signal) including a sync signal, which are respectively supplied to input terminals 11 and 12. The carrier frequency of the carrier chrominance signal Ch supplied to the input terminal 11 is frequency-interleaved with the Y signal supplied to the input terminal 12: EQU 195/2.multidot.fH=1.534091 (Mhz)
where fH is the line frequency. In order to reconvert the carrier chrominance signal of 1.53 MHz into an original carrier chrominance signal of 3.58 MHz, the carrier chrominance signal of 1.53 MHz is supplied to a frequency converter 13 which multiplies the input signal by a CW (carrier wave) signal of 5.11 MHz (1.53+3.58). The CW signal is supplied from a 5.11 MHz VCO (voltage-controlled oscillator) 14. The multiplication product from the frequency converter 13 is supplied to a band-pass filter 15 of 3.58 MHz band-pass characteristic which produces an NTSC chrominance signal of 3.58 MHz. This chrominance signal from the frequency converter 13 and the Y signal supplied to the input terminal 12 are supplied to a mixer 16 which produces an NTSC color TV signal to an output terminal 17.
A video disc player is subject to time base error of signal components which is caused by wow and flutter of a turntable or the like. Such time base error causes significant jitter in the carrier wave of 1.53 MHz frequency. A video disc player generally has an automatic phase control (APC) loop in order to eliminate such jitter. More specifically, the chrominance signal reconverted to a frequency of 3.58 MHz is supplied to a phase comparator 18 which compares the phase of this input signal with that of a reference signal having a frequency of 3.58 MHz during one burst period. The reference signal of 3.58 MHz is supplied from a 3.58 MHz oscillator 19. A burst gate pulse is obtained from a burst gate pulse generator 21, which receives an output signal from a sync separator 20 which separates a sync signal from the Y signal supplied to the input terminal 12, and which then supplies an output signal to the phase comparator 18. An output signal from the phase comparator 18 is held for a 1-H period by a sample and hold circuit 22, and is supplied to an oscillation frequency control end of the 5.11 MHz VCO 14 through a low-pass filter 23.
A PAL color TV system has, in addition to the luminance signal, two chroma signal components according to which the subcarrier is quadrature-modulated. One (R-Y signal) of the two chroma signal components is inverted for each successive line. A carrier frequency fsc of the PAL system is generally selected to be 4.43 MHz.
FIG. 2 shows an example of a demodulation circuit for demodulating such a
color TV signal. Referring to FIG. 2, a PAL composite signal is supplied to an input terminal 25 and is then supplied to a C-Y separator 26 which produces a Y signal onto a line 27 and a carrier chrominance signal onto a line 28. The carrier chrominance signal is subjected to an addition by an adder 30 and a subtraction by a subtractor 31 of a delayed carrier chrominance signal from a 1-line delay line 29. A B-Y signal appears at the output end of the adder 30, while an R-Y signal appears at the output end of the subtractor 31. These B-Y and R-Y signals are respectively supplied to B-Y and R-Y demodulators 32 and 33. The line 28 is also connected to a local subcarrier oscillator 35 through a burst phase discriminator 34. A reference subcarrier signal of 4.43 MHz is supplied to the B-Y demodulator 32 through a 90.degree. phase shifter 36 and is also supplied to the R-Y demodulator 33 through a line changeover switch 37. Since the R-Y signal changes its polarity or phase for each successive line, the reference subcarrier signal to be supplied to the R-Y demodulator 33 must be inverted by 180.degree. for each successive line. Thus, the line changeover switch 37 has a 180.degree. phase shifter 371 and a changeover switch 372. The changeover switch 372 is switched between a 180.degree. phase shift line 381 and a direct coupling line 382 for each line.
In order to control the switching operation of the changeover switch 372, an output signal from a flip-flop 40 is supplied to the changeover switch 372 as a line changeover switch. The output signal from the flip-flop 40 may be obtained by separating with a sync separator 39 a sync signal from the PAL composite signal supplied to the input terminal 25 and supplying the sync signal to the flip-flop 40.
The signals sync-detected by the B-Y and R-Y demodulators 32 and 33 are supplied to a matrix circuit 41 which produces blue, green and red signals B, G and R, respectively.
When a TV receiver of the PAL system receives an NTSC signal, the luminance signal may be reproduced, albeit with some unnaturalness due to slight differences in the standards; for example, the vertical amplitude is shortened since the NTSC system uses 525 scanning lines while the European
system uses 625 scanning lines. However, the chrominance or chroma signal cannot be received due to the system differences.
In European countries where the PAL system is adopted, it may be desired to receive an NTSC signal with a PAL receiver or to receive a PAL signal with an NTSC receiver. In order to respond to such needs, there has been proposed a method involving satellite communication. In accordance with this method, an NTSC color TV signal is demodulated upon reception by the satellite, is converted into a PAL-system color TV signal, and is received by a PAL-system color TV set. Conversely, a PAL color TV signal is converted into an NTSC color TV signal and is received by an NTSC-system color TV set.
However, when it is desired to reproduce signals on an NTSC-system video disc with a PAL-system color TV, the system adopting a satellite as described above may not easily be applied to equipment due to limited standards, high cost and so on.
One example of an NTSC-PAL color TV signal converter circuit is described in Japanese patent application No. 43-14286. This system uses a means for mixing two chroma signal components modulated on the subcarrier with a signal having a frequency double that of the subcarrier. This system may be suitably adapted when the NTSC and PAL systems involved use the same carrier frequency. However, in practice, the NTSC system has a carrier frequency of 3.58 MHz, while the PAL system has a carrier frequency of 4.43 MHz. This implies a need for frequency conversion.